This research investigates steroid action on a central synapse involved in a simple reflexive behavior. In the hawk moth (Manduca sexta), the steroid hormone 20-hydroxyecdysone (20E) regulates the reorganization of neurons involved in the proleg withdrawal reflex (PWR). The tip of each proleg bears an array of mechanosensory hairs, each innervated by a single sensory neuron (termed a PH-SN). In the CNS, axon terminals from the PH-SNs make direct, excitatory synapses with the accessory planta retractor (APR) motor neuron. In response to the prepupal peak of 20E during metamorphosis, these synapses weaken and APR's dendrites regress from the sensory terminals of the PH-SNs. The cellular mechanisms involved in steroid-mediated synaptic weakening and dendritic regression will be studied in vitro using electrophysiological and morphological techniques. Specific Aim 1 is to develop a coculture system for studying the effects of 20E on synaptic transmission in vitro. Specific Aim 2 tests the hypothesis that 20E triggers dendritic regression in APR directly. Specific Aim 3 tests the hypothesis that 20E alters APR's sensitivity to acetylcholine, the neurotransmitter at this synapse. This research will increase our understanding of steroid action on synaptic structure and function and may lead to important clinical insights into the treatment of neurological disease.